This invention relates to methods and compositions for detecting thyroglobulin in a biological sample. Thyroglobulin, which is secreted from functioning thyroid tissue, is very useful as a tumor marker in the diagnosis of recurrent, residual, and metastatic thyroid cancer. The presence of thyroglobulin in the plasma/serum of patients indicates functioning thyroid tissue. Following surgery and treatment with radioactive iodine, patients with thyroid cancer should be free of all functioning thyroid tissue. Detectable thyroid tissue in these patients indicates the need for further therapy.
However, quantitation of thyroglobulin using immunoassays is problematic. Current methods of detection utilizing immunoassays, or antibody-based assays, to quantitate intact protein in patient serum samples are complicated by several limitations. Immunoassays for thyroglobulin are affected by patient autoantibodies to thyroglobulin, patient antibodies to reagent antibodies, reagent antibody saturation with analyte, and lack of standardization (Hoofnagle, A., and M. H. Wener, Clin Lab Int 8:12-14 (2006)). Patient autoantibodies to thyroglobulin bind to thyroglobulin epitopes, or surface features of the protein, and interfere with reagent antibodies binding to the thyroglobulin analyte in the assay, causing falsely low concentrations of thyroglobulin to be measured by traditional “sandwich” immunoassays and falsely high concentrations of thyroglobulin in radioimmunoassays. Some patients have antibodies to reagent immunoglobulins, also called human anti-mammalian antibodies, which can cause an erroneously positive result. For example, approximately 10% of patients have antibodies to thyroglobulin (Tg) that could potentially interfere with immunoassays (Kloos, R. T., et al., J. Clin Endocrinol Metab 90:5047-5057 (2005); Spencer, C. A., Clin Chem 42:661-663 (1996); Spencer, C. A., et al., J. Clin Endocrinol Metab 89:3702-3704 (2004)). The prevalence of Tg autoantibodies increases to 25% in patients with differentiated thyroid carcinoma (Ericsson, U. B., et al., Clin Immonol Immunopathol 37:154-162 (1985); Spencer, C. A., et al., J. Clin Endocrinol Metab 83:1121-1127 (1998); Okosieme, O. E., et al., Clin Chem 52:729-734 (2005)).
In certain cases, very high concentrations of thyroglobulin saturate the reagent antibodies used in immunoassays and prevent formation of tripartite reagent immunoglobulin-antigen complexes, resulting in falsely low concentrations of thyroglobulin to be measured. Lastly, because all immunoassays use different antibodies generated independently by each company, there is suboptimal standardization of thyroglobulin immunoassays, and results from one immunoassay may not be in accord with the results of another.
Therefore, a need exists for improved methods and compositions for the detection of thyroglobulin in order to more accurately detect residual, recurrent, or metastatic disease in patients afflicted with thyroid cancer.